Powder metallurgical (PM) techniques are well established routes for efficient production of complex metal based components. These techniques are commonly used in applications where alloys based on iron, stainless steel, copper or nickel are required. However, the use of PM techniques where material such as titanium, chromium, niobium and tantalum are required has so far been limited.
One issue that arises with the mass production of metal containing powders is the presence of impurities, which can be difficult to avoid. Thus, U.S. Pat. No. 3,140,170 describes an approach in which the object was to provide a process for reduction of titanium oxides to give a product low in oxygen and alloyed magnesium (derived from the magnesium reducing agent). The described approach involves reducing titanium oxides with magnesium metal in the presence of a magnesium dichloride flux and in an atmosphere of hydrogen. This approach is said to enable an oxygen content as low as 2.23 percent. This still represents a significant content of oxygen impurities, though—indeed, the product is described as “impure titanium metal” and is intended for use as a starting material for electro-refining, which is then needed to prepare “high purity” titanium.
More recent publications concerning metal powder preparation include JPH05299216, which concerns the preparation of a rare earth-based alloy magnetic material. In this approach a rare earth oxide, reducing agent and a metal are mixed, a reduction-diffusion reaction treatment is conducted in a hydrogen-containing atmosphere, and the obtained cake-like reaction product is then cooled, with the atmosphere being switched to an inert gas at 770 to 870° C. Another publication is JPH01168827, which describes a method of preparing chromium powder. The method involves mixing chromium oxide with calcium hydride and then heating under hydrogen. A further publication is US 2009/0053132, which describes the preparation of niobium (or niobium suboxide) powder. This approach involves mixing niobium oxides with a reducing agent, reacting the components at a temperature of 600 to 1300° C. in a vacuum or under inert or hydrogen gas, leaching, and then heating (a second time) to a temperature of 1000 to 1600° C.
The above-mentioned publications generally concern the preparation of metal powders. However, a further class of important product is metal containing products such as metal (or metalloid) carbides, nitrides, borides and silicides. Metal (or metalloid) carbides, nitrides, borides and silicides are required for a variety of industrial purposes. Such compounds should desirably be produced by a process which yields the product as a high quality powder. Thus, for example, processes involving strong exothermic reactions can lead to a degraded product due to uncontrolled sintering. Further, a strong exothermic reaction can reduce the efficacy of the process and require an expensive reactor vessel to contain the reaction.
Typical approaches for the mass preparation of products containing a metal (or metalloid) carbide, nitride, boride or silicide involve first preparing a metal product and then carrying out a further step of reacting that metal with a suitable source of carbon, nitrogen, boron or silicon. However, even if a relatively pure starting material is used, it can be difficult to produce a high quality carbide, nitride, boride or silicide product on an industrial scale. Previously described approaches for preparing metal carbides and nitrides include the following.
JPH03159910 describes an approach of milling transition metal powder and carbon powder without heating. Exothermic reaction produces transition metal carbide.
JP 2010059047 describes approximately spherical particles containing a rare earth nitride for use as a magnetic refrigeration material. The particles are prepared by nitriding spherical particles of the rare earth element (e.g. yttrium or scandium).
CN 102616780 discusses some of the difficulties that can arise when preparing products such as titanium carbide, noting that if direct carbonation is carried out by combining powders of titanium and carbon, the reaction is very fast and difficult to control. Against that background, CN 102616780 describes an approach involving the use a direct current (DC) arc method for preparing titanium carbide nanometer particles. This involves using automatically controlled DC arc plasma equipment, wherein a gaseous mixture containing a carbonic reaction gas, an inert gas and an active gas are introduced in the presence of a titanium anode and a graphite cathode.
More recently, CN 103318855 discusses the preparation of chromium nitride. In this regard, a previous approach of using microwave heating to prepare products of this type is noted, but is said to entail difficulty in removing amorphous carbon residue which persists in the product. It is also noted that a previous approach involving an arc discharge plasma method did not lend itself well to mass production, due to high energy consumption and low production capacity. It is further noted that the usual approaches for effecting nitridation of chromium involve subjecting various chromium materials to an ammonia atmosphere, but that these suffer drawbacks (the production of harmful gases and corrosion of equipment) and lead to a low purity product with a high oxygen content. The document then goes on to describe the preparation of chromium nitride by subjecting high purity chromium powder to flowing ammonia at a temperature of 800 to 1200° C. Even then, though, the oxygen contents in the exemplified chromium nitride products are reported to be 2.38% and 1.63%.
Another reference, namely CN101186300, concerns an approach for preparing a titanium silicide product using microwave radiation. The approach involves (a) selecting a reacting substance system containing titanium and silicon, (b) adding doping materials into that system, (c) mixing and ball milling until the particle size is 5 nm-0.5 mm, (d) putting the mixture into a crucible pot and subjecting it to microwave radiation for 0.1 to 10 hours at 100 to 1500° C. in the presence of a protective gas, and (e) washing, filtering and drying to obtain a doped titanium silicide product. One particular product made from titanium powder, silicon powder and urea is described—after pressing, a mixture of these components is heated under Argon at 900° C. for one hour, prior to washing, filtering and drying. A second product made from powders of titanium dioxide, magnesium and silicon is described too—a mixture of these components is heated to 800° C. under Argon for 0.5 hours. As discussed above, though, it is known from CN 103318855 that this approach of using microwaves can lead to amorphous carbon residue in the product, which can be difficult to remove.
The present invention relates to an improved and cost effective production of metal (or metalloid) carbides, nitrides, borides and silicides, whereby these products can be obtained efficiently as high quality powders, directly from a corresponding oxide of the metal (or metalloid). The approach of the present invention also lends itself particularly well to mass production on an industrial scale.